1. Field of the Invention
The present invention relates to photoconductive detectors of light and particularly to a multiband photoconductive detector of light, based on layered semiconductor quantum wells, which can simultaneously and separately detect and distinguish light from a large number of different wavelength bands.
2. Description of the Prior Art
Photoconductive detectors based on semiconductors detect radiation, or light, whenever its photon energy exceeds the forbidden gap energy of the particular semiconductor compound or alloy of which the detector is constructed. For example, an InSb photoconductor can detect light with a wavelength shorter than about five micrometers and detectors based on the HgCdTe alloy system have long wavelength thresholds which can be tailored in the near infrared to infrared range by choice of alloy composition.
To increase the optical absorption of photons with energy above some preselected wavelength threshold, photoconductor detectors have been implemented with multiple layers on a single structure. An example of such multiple layers is disclosed in the published article of R. Dingel et al., "Quantum States of Confined Carriers in Very Thin Al.sub.x Ga.sub.1-x As-GaAs-Al.sub.x Ga.sub.1-x As Heterostructures", Physical Review Letters, V. 33, No. 14, pp. 827-830 (Sept. 30 1974). This article reports that, to increase the GaAs optical absorption, as many as fifty GaAs layers have been grown on a single substrate, and that these GaAs layers (of a thickness between 40 and 50 angstroms) were separated by Al.sub.x Ga.sub.1-x As layers (of a thickness normally greater than 250 angstroms).
In many multi-layer applications it is desirable not only to detect indiscriminately all photons with energy above some threshold, but to be able to detect separately the incident light in specific wavelength bands. It is known that multi-wavelength band detection is presently being done with the HgCdTe alloy system by changing the alloy composition (or amounts of mercury (Hg) and cadmium (Cd)) to change the various bandgaps (or frequency windows).
A multi-layer photoconductive detector implemented by changing the alloy compositions possesses several disadvantages. It is crude, relatively difficult and time-consuming to grow with different alloy compositions.